1. Field of the Invention
This invention relates to a method for driving, for example, an on-demand type ink jet recording head which is used in an ink jet recording device and discharges ink droplets by applying a driving voltage to an electromechanical transducing element of the head.
2. Related Background Art
FIG. 1 is a schematic upper view of an example of an ink jet recording device in general, FIG. 2 of a block diagram showing its control system.
In FIG. 1, numeral 1 is a platen which rotates in predetermined increments to enable sub-scanning during the recording of a recording medium (not shown) wound therearound. Numeral 2 is a line feed motor which rotates to the rotational shaft of the platen 1 through a gear 3. Numeral 4 is an ink jet recording head (hereinafter called "head") freely slidable on a guide bar (not shown) arranged parallel to the platen 1. The head is provided with a plurality of discharge openings or nozzles 5 for discharging ink as droplets. Numeral 6 is a belt for moving the head 5 reciprocally in the longitudinal direction of the platen 1, numerals 7 and 8 are pulleys arranged at the both ends of the belt 6, and numeral 9 a carriage motor for rotating the pulley 8.
Numeral 10 is a paper sensor for detecting the presence of recording medium arranged in the vicinity of the surface of the platen 1, numeral 11 an encoder sensor mounted on the head 4, and numeral 12 a linear encoder arranged parallel to the platen 1 and also opposed to the encoder sensor 11. Numeral 13 is a home position sensor for detecting that the head 4 is in the home position, numeral 14 is a cap which is used when restoring poor discharge including non-discharge. Numeral 15 is a motor which is the driving source for moving the cap 14 forward and backward with respect to the head 4, and numeral 16 a cap sensor for detecting that the cap 14 is mounted on the head 4.
In the above constitution, when the recording medium is mounted on the platen 1, the paper sensor 10 detects whether it is in a recording position. When the recording start button is pushed, the carriage is moved, and the head 4 moves from the home position following the printing format of the recording device, and permits ink droplets to fly from the discharge opening to reproduce the recording data. The head 4 is subjected to main scanning, driven by the belt 6 with the motor 9 as the driving source. Every time one line of main scanning is completed, the motor 2 is driven to rotate the platen 1.
To prevent clogging of the discharge openings of the head 4, the cap 14 is positioned to cover the head 4 periodically or if necessary. This state is detected by the cap sensor 16, which then interrupts the process. The restoration process comprises absorbing the ink within the nozzles by an absorbing mechanism (not shown) within the cap 14, thereby removing foreign matter etc. within the nozzles. By doing so, the restoration process prevents any defective recording.
Next, the constitution of the control system shown in FIG. 2 will be described.
CPU 20 constitutes the main body of control, to which are connected a group of switches 21 (arranged on the operational panel) through an input and output interface (not shown), a DC servo reversing circuit 22 for driving the carriage motor 9, a stepping motor driving circuit 23 for driving the line feed motor 2, a head driver 24 for driving the recording head 4 based on the recording data, a group of various sensors 25, the encoder sensor 11 and the home position sensor 13.
In the constitution shown in FIG. 2, CPU 20 performs the following operational steps corresponding to the operational input performed by the switch group 21 provided on the operational panel (not shown). More specifically, by referring to the input from the encoder sensor 11 and the home position sensor 13, the driving control of the carriage motor 9 is conducted through the DC servo reversing circuit 22, and also the driving control of the line feed motor 2 through the stepping motor driving circuit 23, whereby the recording data D are outputted to the head driver 24 to drive the recording head 4. Also, control of the other mechanisms corresponds to the inputs from another group of sensors 25.
Under this constitution, the recording process is commenced by actuating the print switch of the switch group 21. The line feed motor 2 is then driven several steps, on confirmation of the presence of recording paper by the paper sensor 10, to rotate the platen 1 and set the recording paper at the recording start position. Subsequently, the carriage motor 9 is driven to move the recording head 4 in a reciprocating manner, and the line feed motor 2 is driven as synchronized therewith to deliver the recording paper line by line. During such actuation, driving signals corresponding to the recording data are applied from the head driver 24 to drive the recording head 4, whereby ink droplets are discharged through the openings of nozzle 5 to effect recording of letters, images, etc.
FIG. 3 shows a schematic perspective view of a head unit including the nozzle of the head 4 in FIG. 1. At the tip end of the tubular ink liquid path 41, a tapered nozzle 42 is formed. On the outer surface of the nozzle 42 near the discharge opening 5, a piezoelectric element 43 for generating energy used for discharging ink is externally positioned. Also, within the inlet of the ink liquid path 41, a filter 44 is inserted to excluded foreign matters, impurities, etc. To the piezoelectric element 43 a head driver 24 is connected through a lead wire.
In the constitution in FIG. 3, ink is filled in the ink liquid path 41, and when a predetermined driving voltage is applied by the head driver 24 on the piezoelectric element 43, the piezoelectric element 43 creates a strain, thereby generating pressure in the ink liquid path 41 to discharge the ink droplets 47 from the discharge opening 5.
In this case, as shown in FIG. 4, in response to the input signal, first a voltage Vrev of negative polarity is generated for a time of T1, which voltage is applied to the piezoelectric element 43 to expand the ink liquid path 41. Next, a positive voltage Vop is generated for a time period T2, which is applied to the piezoelectric element 43 to reduce the ink liquid path 41, thereby discharging the ink as droplets 47. Further, the application voltage is gradually reduced over a time period T3, thereby effecting restoration actuation of the nozzle diameter. By setting suitably the levels of the voltages Vrev, Vop or the time period T1, T2, the ink discharging amount can be varied. For example, (1) ink droplets of greater diameter can be discharged as the time period T1 is increased corresponding to the ink discharging amount. Also (2), with Vrev equal to zero volts, and by varying the voltage Vop or the time period T2, the ink droplet discharging amount can be effectively varied.
However, in such a recording method of the prior art, in the case according to the discharging control method of example (1) as described above, when ink droplets with large diameters are desired to be discharged, the pressure change within the ink liquid path must be made very great, whereby small bubbles are generated near the filter portion and the ink discharging can be maintained stably with difficulty.
On the other hand, in the case according to the discharging control method of the above example (2), no sufficient dynamic range from ink droplets with small diameters to ink droplets with large diameters could be obtained.